Ionization vacuum gauges, like all electrical vacuum gauges, measure pressure as the particle count density. For this purpose a portion of the molecules or atoms in the gas chamber is ionized. The ions thus produced yield their positive charge to a measuring electrode of the system. The ion current thus produced is an index of the pressure. The ions are formed, as a rule, by electron collision. For this purpose the electrons are emitted by a thermionic cathode.
Since the thermionic cathode of such systems can be operated only at pressures of less than about 10.sup.-2 millibars on account of the fragility of the thermionic emission coating, any defect in the measuring system will not be detected until after a relatively long evacuation process has already been performed. In the case of such a defect, the cell then must be vented for the repair of the gauge, and then the evacuation process must be repeated.
The object of the present invention is to design the power supply circuit of the ionization vacuum gauge such that, whenever the cause of a fault in the gauge system is a break in the thermionic filament, this defect can be detected before the evacuation process begins.